1. Field of the Invention
The present invention relates to an injection molding method to be applied to forming a molding, such as a half case of a video tape cassette or an audio tape cassette, by sequentially injecting different molding materials into a die cavity formed between a male die and a female die.
2. Description of the Prior Art
A half case of a video tape cassette or an audio tape cassette is formed by a two-color injection molding method of either a rotary system or a core-back system.
3. Rotary System
In forming a molding by a two-color injection molding method of a rotary system, a core die is turned in a plane perpendicular to a female die by a die inverting device to dispose the core die alternately opposite to a first cavity die and a second cavity die for two-stage molding. The two-color injection molding method of a rotary system needs two dies, i.e., the first cavity die and the second cavity die, needs a long cycle time because the die must be opened after the completion of molding in the first cavity die to turn and transfer the first molding to the second cavity die and hence has a low productivity. Since the die is opened after the completion of the first molding cycle and a first molding is exposed to air before forming a composite molding by combining a second molding formed in the second molding cycle, the composite molding is liable to separate into the first molding and the second molding along the interface between the first and second moldings.
4. Core-back System
A two-color injection molding method of a core-back system has been proposed to solve the problems in the two-color injection molding method of a rotary system. A typical two-color injection molding method of a core-back system is disclosed in Japanese Unexamined Patent Publication (Kokai) No. Sho 63-293024 (Method of Molding a Two-color Half Case of a Tape Cassette). The gist of the invention disclosed in Japanese Unexamined Patent Publication (Kokai) No. Sho 63-293024 will be described hereinafter with reference to FIG. 18, which is a copy of FIG. 1 included in the specification of the same invention.
In injecting a first material into a cavity formed in a die consisting of female die 110 and a male die 115, a slide core 126 for forming windows in a half case of a tape cassette, movable on the male die 115 is projected toward the female die 110, a cavity core 122 movable on the female die 110 is projected so as to close spaces for the windows and the surface of a rim, and the first molding material is injected into a cavity formed between the female die 110 and the male die 115. Then, the slide core 126 and the cavity core 122 are retracted to form spaces to be filled up with a second molding material, and then the second molding material is injected into the spaces to complete a half case.
An invention disclosed in Japanese Unexamined Patent Publication (Kokai) No. Sho 57-187228 proposed to eliminate disadvantages in this two-color injection molding method of a core-back system that dimples are formed in the surface of the molding and the die is subject to damaging employs a slide core 5 provided with a recess 16 in its contact surface facing a gate as shown in FIG. 19a and 19b, which is a copy of FIG. 4 included in the specification of Japanese Unexamined Patent Publication (Kokai) No. Sho. 57-187228.
The die employed in the two-color injection molding method of a core-back system has a complicated construction, which affects adversely to molding efficiency and causes the following problems.
1. Low productivity
Since the die has a complicated construction, it is difficult to form an effective cooling passage in the die, the core-back system needs a long cycle time, troubles occur frequently during the process and the reliability of the die is not high enough.
2. Low economic effect
The die having a complicated construction is expensive and requires much time for fabrication. Nevertheless, it is difficult to obtain a die capable of satisfactorily improving the productivity.
3. Low degree of freedom of design
There are many restrictions on the design of the die to give high priority to reliability and it is difficult to incorporte improvements into the die.
The foregoing method of preventing defects attributable to a gate drop has the following problems.
1. Problems in commercial value
Since the depth of the recess formed in the contact surface of the slide core facing the gate must be considerably large to avoid crushing the gate drop by the slide core, grooves having a pattern spoiling the design of the half case are formed around the transparent portion of the half case, for example, a portion around the windows.
2. Problems in strength
The wall thickness of the portion of the half case in which the grooves are formed in an undersirable pattern around the windows is smaller than that of other portions of the half case, which affects adversely to the strength of the half case.
3. Problems in production
In some cases, the die was broken due to fatigue when the width of the contact surface was increased to attach importance to design and the wide contact surface obstructed the flow of the molten resin to deteriorate moldability.